This invention relates to the manufacture of electrode assemblies for electrochemical cells and, more specifically, to the testing of the joint formed between the electrode assembly for the cell and a terminal conductor attached to one of the cell electrodes.
Rechargeable electrochemical cells are made in many condfigurations. Most sealed electrochemical cells, however, are made by forming an electrode assembly comprised of interleaved positive and negative electrodes each separated from the other by a separator layer which is adapted to absorb and hold a liquid electrolyte, a metal container which serves as the negative terminal for the cell, a top cover which generally forms or contains the positive terminal for the cell, and a terminal conductor which electrically interconnects the positive electrode with the positive terminal of the cover or cover assembly.
There are several methods for connecting the terminal conductor between the electrode and external terminal. For example, a conductor tab can be formed integrally with the positive electrode, or it can be riveted or spot-welded to the edge of the electrode. Although this method of connection can be satisfactory, it is advantageous to have the terminal conductor contact the positive electrode at several points so as to obtain a better current distribution in the electrode by withdrawing current from the electrode at a few, and preferably several, points rather than at a single point as in the case of an integrally formed conductor tab. When the terminal connection is formed to provide several contact points with the electrode, it can be applied to the spirally-wound electrode assembly by percussion welding. A typical electrode assembly having attached thereto a terminal conductor tab is described in U.S. Pat. Nos. 3,695,935 and 4,029,856, both assigned to General Electric Company.
In some cell manufacturing methods, it has been the practice to weld the conductor tab to the exposed edges of the electrode assembly while it is held in a supporting nest used for winding electrodes. Thus, the individual positive and negative electrodes, together with a separator, are fed into the nest and wound by a retractable mandrel into the form of a spirally-wound coil. This nest is advanced stepwise from one position to the next along an assembly line where, ultimately, the electrode assembly is pushed into a metal casing. At one of the intermediate stations between the winding operation and the loading of the electrode assembly into the casing, the conductor tab is welded to the exposed edges of the positive electrode plate. Welding at this point in the manufacturing procedure is advantageous because the electrode assembly is already held in place and oriented to present to the welder the exposed electrode edges of the electrode.
Notwithstanding the use of automated transporting and welding equipment, a certain percentage of cells turn out defective. Operational defects are often traceable to a faulty weld joint between the conductor tab and the electrode. Even when the conductor tab is physically attached to the electrode, cell performance can vary from an acceptable standard. For example, "cold" joints can be encountered, bent or malformed conductor tabs can preclude the formation of a proper weld at all points desired, or welding at fewer than the desired number of crosspoints between the conductor and the electrode might result, thus producing variable internal cell impedance.
In the past, there was no satisfactory method or means for quickly checking the welds on each electrode assembly. Therefore, standard quality control methods were implemented. For example, only a small percentage of the cells were singled out for inspection and measurement. However, this inspection was usually visual only and, although it could isolate obvious weld defects, it did not expose more subtle weld problems such as the "cold" joint and insufficient weld area. Moreover, as stated above, visual inspection was done only on a sampled basis and would never detect faults with each and every cell.
In addition to visual inspection, it is the usual practice to electrically test each cell produced by applying a known current pulse to the cell and measuring its terminal voltage during and after the pulse. This electrical test is highly reliable; however, it is not conducted until much later in the manufacturing process, when the cell is already completed. Sometimes several tens, hundreds, or even thousands, of cells are manufactured before the first defective cell reaches the final performance test. If the defects are not detected prior to final assembly of the cells, all of the produced cells could be useless. On the other hand, although electrical continuity tests can be performed on the conductor tab-electrode assembly weld prior to final assembly, such electrical tests do not realiably reveal insufficient mechanical integrity of the weld. Even if a defective weld is detected, moreover, the conductor tab must be removed by hand in order to reuse the partially completed unit.
The present invention is aimed at a solution of the problem of controlling the quality of the production of electrochemical cells by early testing of the terminal connection between the electrode assembly and terminal conductor. In accordance with the invention, the integrity of the joint between the conductor and electrode assembly is tested by applying to the conductor, for a given time, a predetermined minimum tensile force tending to separate the conductor from the electrode assembly so as to establish thereby a minimum acceptable stress on the joint being tested. Any movement of the terminal conductor as a result of this force is detected, and an indication of unacceptability is provided when any such movement exceeds a predetermined distance corresponding to an insufficient mechanical integrity of the joint.
In the preferred embodiment to be described, a detected excess movement of the terminal conductor results in an electrical signal which causes the applied tensile force to increase to a level sufficient to completely separate the conductor from the electrode assembly, thus automatically removing the conductor defectively attached to the electrode assembly and permitting the recycling of the assembly in the manufacturing process. Equally important, however, is the fact that the defective attachment is detected substantially immediately.